Transition-metal-catalyzed asymmetric synthesis is an important area in modern organic chemistry study (Ohkuma, T; Kitamura, M.; Noyori, R. Catalytic Asymmetry Synthesis, Wiley, New York, 2000). The crux of the asymmetric catalytic synthesis is to synthesize the chiral catalyst. Since the asymmetric control emerged by chiral catalyst which depends on chiral ligands, for the core of the synthesis of chiral catalyst is the synthesis of chiral ligands. The motivation of the asymmetric synthesis development is to design and synthesize the new chiral ligands.
Chiral carboxylic acid, one of the important blocks for synthesis, is widely applied into synthesis of drugs and nature products. Wherein alpha-aryl substituted propionic acid, such as naproxen and ibuprofen, is widespread used in the world today as a non-steroidal anti-inflammatory drug. There is significant application value to develop the synthesis method of optical active alpha-substituted propionic acids with high efficiency and high selectivity. To obtain the compound, transition-metal-catalyzed asymmetric hydrogenation of alpha-substituted acrylic acids is one of the most direct and effective method. In the past several decades, much research on transition-metal-catalyzed asymmetric hydrogenation of alpha-substituted acrylic acids has been done and some valid chiral ligands and catalysts are developed. Among the research of asymmetric hydrogenation of alpha-substituted acrylic acids so far, axial chiral diphosphine ligands and the complex catalyst of ruthenium acquire the best results. Nevertheless, to maximize the catalytic effect for such catalyst requires hydrogen pressure greater than 6 MPa in general. If hydrogen pressure is reduced, the activity and enantioselectivity of reaction is lowered (1. Ohta, T.; Takaya, H.; Kitamura, M.; Nagai, K.; Noyori, R. J. Org. Chem. 1987, 52, 3174; 2. Chan, A. S. C.; Laneman, S. A. U.S. Pat. No. 5,144,050, 1992; 3. Benincori, T.; Brenna, E.; Sannicolò, F.; Trimarco, L.; Antognazza, P.; Cesarotti, E.; Demartin, F.; Pilati, T. J. Org. Chem. 1996, 61, 6244; 4. Pai, C.-C.; Lin, C.-W.; Lin, C.-C.; Chen, C.-C.; Chan, A. S. C. J. Am. Chem. Soc. 2000, 122, 11513; 5. Qiu, L.; Wu, J.; Chan, S.; Au-Yeung, T. T.-L.; Ji, J.-X.; Guo, R.; Pai, C.-C.; Zhou, Z.; Li, X.; Fan, Q.-H.; Chan, A. S. C. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5815). The reaction condition of high pressure has a high requirement of apparatus and causes many security problems in production. Diphosphine ligands and complex catalyst of rhodium are also used in asymmetric hydrogenation alpha-substituted acrylic acids. Although some rhodium catalysts have relatively high enantioselectivity in asymmetric hydrogenation alpha-substituted acrylic acids, the catalyst dosage is large (1 mol %) and the enantioselectivity in substrates of alpha-alkyl substituted acrylic acids is only middle class which is not adequate enough (1. Robin, F.; Mercier, F.; Ricard, L.; Mathey, F.; Spagnol, M. Chem. Eur. J. 1997, 3, 1365; 2. Hu, W.-H.; Pai, C. C.; Chen, C. C.; Xue, G.-P.; Chan, A. S. C. Tetrahedron: Asymmetry 1998, 9, 3241; 3. Zupan{hacek over (c)}i{hacek over (c)}, B.; Mohar, B.; Stephan, M. Org. Lett. 2010, 12, 3022). Recently, some phosphine-oxazoline ligand and complex of iridium are used to catalyze the asymmetric hydrogenation of alpha-substituted acrylic acids. However, since the conversion number (TON<100) and transformation frequency (TOF<13 h−1) is relative low and the highest enantioselectivity is only 88% ee, the requirement of practical application can not be fulfilled (1. Scrivanti, A.; Bovo, S.; Ciappa, A.; Matteoli, U. Tetrahedron Lett. 2006, 47, 9261; 2. Zhang, Y.; Han, Z.-B.; Li, F.-Y.; Ding, K.-L.; Zhang, A. Chem. Commun. 2010, 46, 156). In conclusion, with regard to asymmetric hydrogenation alpha-substituted acrylic acids, the existing chiral ligands and catalysts have defects of high hydrogen pressure, large catalyst dosage, long duration of reaction and limited substrates, which have negative effect on practical application. Therefore, to overcome the shortages of existing ligands and catalysts is one of the focuses of the research to develop new effective chiral ligand and corresponding catalyst.